Two-stroke engines are commonly provided intake and exhaust ports in the cylinder wall through which intake gases are provided into the cylinder and exhaust gases are scavenged from the cylinder. The ports are covered and uncovered by pistons reciprocating in the cylinder and the open durations of the intake and exhaust ports overlap. During overlap and at some operating conditions, the respective pressures in the cylinder and the intake system are such that exhaust gases from the cylinder backflow through the intake ports into the intake system thereby negatively impacting scavenging effectiveness of the engine. It is known to provide reed valves in the intake system as close to the intake ports as possible to largely prevent backflow of exhaust gases into the intake system. Reed valves allow flow from the intake system into the cylinder; however, the valves close off flow when pressure in the cylinder is greater than the pressure in the intake, thereby preventing back flow into the intake. Reed valves, unfortunately, present a pressure drop in the intake system and thus reduce the peak torque that the engine may develop. It is desirable to improve scavenging effectiveness, but without increasing pressure drop in the intake.
It is desired to provide a larger intake port area than exhaust port area because the driving pressure on the exhaust gases is greater than the driving pressure on the intake gases. In an engine in which the ports are provided substantially evenly around the periphery, the total intake port height is greater than that of the exhaust port height to provide the desired flow area. However, such greater intake port height is inconsistent with the desired port timing in which the exhaust ports open and close prior to the opening and closing of the intake ports.